As cannabis is not water soluble and nicotine (+ many other none cannabinoids) is it is advisable to always smoke cannabis by pulling it through water. In many situations this is not available (prison , outdoors etc) and the user has to turn to something like a Lung to get the job done.
Usually a lung is made from a plastic bottle 2lt or larger with the bottom cut off and a plastic bag taped taped to the bottom of the bottle in its place. If the bag is airtight it is pushed upwards inside the bottle and a head (13ml socket set + gauze) is placed into the top of the bottle then filled with whatever the user is smoking.
The inflation of the lung is usually a co-operative process involving 2-3 people as the bottle needs to be held upright to stop the head from falling out while the bag is pulled from the bottom into an inflated position while a lighter is held over the top of the head to light the smoking material. The action of manually inflating the lung draws large amounts of air into the the plastic bottle filling the plastic bag. The smoker then has to remove the head (ouch its hot >.< ) and attempt to suck the entire contents of the bag into their lungs.
(though it can be operated by a single person by attaching the bottom...
it is used by putting the bag up inside the bottle and lighting the weed in the bowl, while you are lighting you pull the bag down, causing suction, while doing fills the lung with smoke.
Lungs get you very fucking stoned.